Low coherent interference technique for the demodulation of displacement information mainly comprises two methods, that is, time scanning and spatial scanning. In the time scanning method written by M. Song et al., (M. Song, Byoungho Lee, An effective optical evaluation technique using visible low-coherence interferometer, Optics and Lasers in Engineering, 1997, 27: 441-449), the displacement information is obtained by scanning the optical path difference of one of the arms of Michelson interferometer. In the spatial scanning method, the displacement information is converted into a linear distribution of spatial optical path difference and received by a CCD linear array. In order to obtain the spatial distribution of optical path difference using the above stated scanning methods. Chen et al. proposed a Fizeau interferometer in 1991 and Marshall et al. proposed a Mach-Zehnder interferometer in 1996 (S. Chen et al., Study of electronically-scanned optical-fiber white-light Fizeau interferometer, Electronics letters, 1991. 27(12):1032-1034; R. Marshall et al., A novel electronically scanned white-light interferometer using a Mach-Zehnder approach, Journal of Lightwave Technology, 1996. 14(3): 397-402).
The time scanning method is capable of measuring displacement in a large scope, but its long-term reliability and stability is poor. In addition, the accuracy of the obtained data is limited to micron level. The space scanning method has the advantages of long-term reliability, by using linear CCD array for electric scanning. However, the scanning light beam covers entire CCD photosensitive surface, which results in a spread of light energy and low signal-noise ratio.